As well paths in exploration and extraction activities in the oil and gas industries become increasingly longer and the network more complicated, new challenges are constantly faced in the area of well boring. One of these challenges is in respect of maintaining suitable conditions for the operation of the drill head or bit to cut through the medium.
When drilling well bores, a slurry, used to operate the drill head, is pumped from the surface through a drill stem assembly to the drill head. Upon reaching the end of the bore the slurry is caused to return to the surface, passing through the annular space between the sidewalls of the bore and the drill stem assembly before reaching the surface. The returning slurry suspends the cuttings made during the drilling process, transporting them back to the surface. The removal of the cuttings ensures the bore remains relatively clear, providing the drilling process with optimal conditions. Failure to remove the cuttings, or a suitable percentage of the cuttings, can cause equipment failure, resulting in costly down time and may even result in the equipment being irretrievable.
A build up of cuttings may result due to a reduction in the flow of slurry to and from the drill head, it may place unnecessary loads on the drill head and stem assembly, and may also cause the drill head to be wedged or jammed in the bore. Furthermore, with the increase in use of horizontal bores in well networks, a build up of cuttings on the lowermost, or bottom surface of the bore may cause side displacement of the drill head, resulting in the bore being drilled in a new and incorrect direction.
Several areas of the drilling process have been analysed and improvements have been made which enhance the process of returning the cuttings to the surface. These solutions have included improvements to the slurry composition used to drive the drill head, as well as improvements to the actual drill head and the speed in which it may operate. However, improvements are still required in order to increase and improve the removal of cuttings from the bore.
Increasing the flow rate of the slurry and hence the return rate of the slurry to the surface does not sufficiently solve the problem. Owing to the narrow gap of the annular space, cuttings still tend to collect in this area. This not only inhibits the drilling process but is also problematic when introducing pipe linings. Also, during subsequent cementing, additional problems are experienced if the hole is not relatively clear.
Several mechanical flushing devices have been developed to assist in the clearing of the cuttings. These devices are incorporated with the drill stem assembly and, typically, when in a closed condition, allow the passage of the slurry to pass therethrough before proceeding to the drill head. When required the flushing device is caused to move to an open condition. In this condition a percentage of the slurry is diverted from the main flow passage through ports located on the outside of the flushing device and passes into the annular space between the flushing device and the bore wall. The flushing device is remotely operated from the surface and typically requires the surface pumps to be switched off before the condition of the flushing device may be changed.
As the annular space between the drill stem assembly and the bore wall is particularly narrow, it is often the cause of packing or wedging of cuttings in this region. It is therefore highly desirable to keep this region clear. Hence, diverting a portion of the fluid as it passes through the flushing member to the annular space assists in flushing this area and maintaining a clear passage for the flow of return fluid and cuttings to the surface.
Several of these flushing devices are referred to in U.S. Pat. No. 6,161,632. The invention disclosed in U.S. Pat. No. 6,161,632 provides a flushing device which remains in a closed state by the weight of the drill stem acting downwardly thereupon. Relieving this weight by applying a pulling force upon the drill stem results in the flushing device moving to an open state, allowing a predetermined percentage of the slurry to be diverted from the main passage into the annular space. The slurry continues to be diverted for as long as the weight of the drill stem assembly has been relieved.
Subsequent, to the return of the weight loading, the ports close and the full flow of the slurry is again delivered to the drill head allowing the drilling process to continue.
Hence, in order to maintain the flushing device in a closed state a compressive force must be maintained upon the flushing device. Similarly, in order to maintain the flushing device in an open state a tractive force must be maintained upon the flushing device. If the compressive or tractive force is not constantly applied to the flushing device, the flushing device may automatically, and undesirably move to the alternate condition.
An outer valve part and inner valve part of the flushing device disclosed in U.S. Pat. No. 6,161,632 are connected in permanent rotation via key and keyway slots between the valve parts. The manner in which these parts are coupled together result in high concentrations of forces at this junction. These concentrations often lead to failures at this interface, requiring costly repairs to be made.
A further deficiency in this device is in relation to the bleed holes located on the outer wall of the tool. These bleed holes allow cuttings to ingress into the flushing device, contaminating the various parts within the flushing device, which may result in tool failure.
Another type of flushing device currently available overcomes some of the deficiencies of the device disclosed in U.S. Pat. No. 6,161,632 in that a tractive force does not need to be maintained upon the device in order for it to remain open. However, a compressive force must be maintained in order to keep the device in a closed state. Lose of compression will automatically result in the tool moving to an open state, regardless of whether the surface pump(s) are in operation or not. This is undesirable during a drilling operation. Furthermore, this particular device is constructed such that the major moving components operate in the slurry, leading to reliability problems. Also, similar to U.S. Pat. No. 6,161,632, breather ports located on the apparatus allow cuttings to enter the device, causing additional problems in relation to the reliability of the device.
This prior art device also relies on hydraulic pressure to achieve the required tensile loading. Owing to the configuration of the device, hydraulic lock can occur resulting in a build up of pressure. When the pressure builds beyond a predetermined value, a safety relieve disc bursts causing the device to default to an open state. The drill stem assembly must then be brought back to the surface so that the flushing device can be repaired or replaced.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.
It is an object of this invention to provide to ameliorate, mitigate or overcome, at least one disadvantage of the prior art, or which will at least provide the public with a practical choice.